1. Field of the Invention
The present invention is a saponified ethylene-vinyl acetate resin having good melt extrusion stability, drawdown resistance, interlayer adhesion and gas-barrier properties, and products made therefrom having good surface smoothness.
2. Description of the Related Art
Saponified ethylene-vinyl acetate resins (hereinafter referred to as EVOH), have good gas-barrier properties and melt processability, and may be melt processed into films, sheets, pipes, tubes, bottles and other types of products. These products are particularly useful as packaging materials for food, and for packaging materials that are required to have good gas-barrier properties.
However, products made from conventional EVOH resin often exhibit “streaks” running in the extrusion-processing direction which degrade the appearance of the products. This is a long-standing problem which has assumed great importance in the field of extruding EVOH materials, because these streaks significantly detract from the commercial value of EVOH products which have them. The degradation in appearance caused by such “streaks” differs from that caused by “skin roughness”, “fish eyes”, “hard spots” and other discontinuities appearing on the surfaces of EVOH products, which are discussed in JP-A-197603/1986 and JP-A-71620/1997. Specifically, these streaks appear on the surfaces of EVOH products, and are nearly continuous in the extrusion direction.
While “streaks” are most often seen in single-layered EVOH extruded products, they are also observed when multi-layer extrusion products, containing both EVOH layers and layers formed by other thermoplastic resins. In addition, streaks can appear immediately from the start of extrusion processing, but may often not be apparent at the beginning of an extrusion process, but may then appear sometime later in the process.
EVOH also has other technical problems:
The load on the extruder varies during the extrusion of EVOH, thereby causing fluctuations in the product thickness;
When EVOH is processed into films or sheets or into parisons for pipes or bottles by extrusion, the product often exhibits “drawdown” from the die (i.e., a reduction in width and/or thickness of the polymer melt upon exiting the extruder die);
When EVOH is co-extruded with other thermoplastic resins, the resulting products do not always exhibit good interlayer adhesion, causing delamination of the layers of the co-extruded product.